I work on a busy, highly transactional database server running SQL Server 2008 R2. There were performance problems last year which were wrongly attributed to SAN replication. Replication was switched off, the problems persisted, application changes were made which resolved the problems then Syncronous SAN Replication was switched back on.

WRITE_LOG operations, as a percentage of overall wait time statistics, went from 20% to 50% as a result of re-enabling SAN replication. There's a battery backed write cache on the SAN but Disk Queue Lengths are up a bit and other PERFMON counters show a negative impact. The system is coping with SAN Replication on but there is definitely a cost associated with it, mainly to log writes.

This article - http://blogs.msdn.com/b/psssql/archive/2013/04/22/how-it-works-always-on-when-is-my-secondary-failover-ready.aspxexplains how 'Log Blocks' are replicated in AlwaysOn artictures, rather than discreet transactions, to maintain a syncronized state.

We are about to implement a new (again highly transactional) system using SQL Server 2012 AlwaysOn with syncronous replica's at a DR site, local clusters at each site using shared storage, AlwaysOn data mirroring to the DR site instead of SAN replication.

This and the PSS article compliment each other for certain architectures, both articles are great.Thanks

Paul Brewer (4/24/2013)WRITE_LOG operations, as a percentage of overall wait time statistics, went from 20% to 50% as a result of re-enabling SAN replication. There's a battery backed write cache on the SAN but Disk Queue Lengths are up a bit and other PERFMON counters show a negative impact. The system is coping with SAN Replication on but there is definitely a cost associated with it, mainly to log writes.